1. Field of the Invention
The present invention relates to a method and apparatus for multi-beam optical scanning, and more particularly to a method and apparatus for multi-beam optical scanning capable of generating multiple scanning beams for forming multiple latent images in a multicolor image forming apparatus and effectively adjusting scanning line pitches of the multiple scanning beams.
2. Discussion of the Background
In recent years, a tandem-type color image forming apparatus has been developed, including an optical scanning system that emits multiple beams for basic printer colors in sequence. The optical scanning system uses a plurality of laser light sources for generating the color corresponding multiple beams and a plurality of optical components sets, which are integrated in a single-piece unit and are put into a single housing. The apparatus includes plural sets of a photosensitive member and a development unit for forming and developing basic color toner images. The basic color toner images formed on the photosensitive members are transferred one after another at a substantially same position on a recording sheet.
Based on this apparatus, various color image forming apparatuses including color printers, digital color copying machines, and so on have been developed, which includes four photosensitive drums arranged in parallel along a sheet transfer direction and in combination with four development mechanisms for magenta, cyan, yellow, and black colors.
The above-mentioned apparatuses have usually required a relatively large space to accommodate the plurality of optical scanning systems for generating multiple laser beams. Japanese Laid-Open Patent Application Publication No. 04-127115 proposes a structure for an optical scanning system in which a single light deflection device is used and plural sets of optical scanning elements are stacked in the sub-scanning direction.
FIG. 1 is a top view of a background multi-beam optical scanning apparatus having four optical scanning systems sharing a single light deflection device although only two of them are seen in FIG. 1. In FIG. 1, reference number 1 denotes a light source, reference numeral 2 denotes a coupling lens, reference numeral 3 denotes an aperture element, reference numeral 4 denotes a first imaging optical device set, reference numeral 5 denotes a light deflection device, reference numeral 6 denotes a second imaging optical device set, and reference numeral 7 denotes a photosensitive member.
In each of the optical scanning systems, the first imaging optical device set 4 changes a laser light beam emitted from the light source 1 and passing through the coupling lens 2 and the aperture element 3 into a line image extended in a main scanning direction in the vicinity of a deflective reflection surface of the light deflection device 5. The second imaging optical device set 6 changes the laser light beam deflected by the light deflection device 5 into a laser light spot on a surface of the photosensitive member which serves as a member for carrying an image.
In the above-mentioned background multi-beam optical scanning apparatus shown in FIG. 1, the laser light beam emitted from the light source 1 is changed into a desired shape by the coupling lens 2 and, after passing through the aperture element 3, the laser light beam is processed through the first imaging optical device set 4 to form a line image extending in the main scanning direction in the vicinity of the deflective reflection surface of the light deflection device 5. The laser light beam is deflected in directions within a predetermined angle by the rotation of the light deflection device 5 to scan the surface of the photosensitive member 7 after passing through the second optical imaging device set 6 to form an image thereon. As shown in FIG. 1, the optical scanning systems are arranged on left and right sides in a symmetric formation with the light deflecting device 5 therebetween.
FIG. 2 shows a cross section of the above-mentioned background multi-beam optical scanning apparatus in the sub-scanning direction. In FIG. 2, reference numeral 8 denotes an optical element included in the second optical imaging device set 6, and the optical elements 8 are stacked in the sub-scanning direction.
FIG. 3 schematically shows the structure from the light source 1 to the light deflecting device 5 of the above-mentioned background multi-beam optical scanning apparatus, seen in a direction horizontally traversing the axis of the laser light beams. The laser light beams emitted from the light source 1 are coupled by the coupling lens 2 and pass through the first optical imaging device set 4. After passing through the first optical imaging device set 4, the laser light beams form a line image extending in the main scanning direction in the vicinity to the light deflection device 5.
In the above-mentioned background multi-beam optical scanning apparatus, many optical devices are made of plastic materials. While plastic materials are superior for mass production, they often fail in forming a desired shape due to an uneven temperature provided inside a mold tool during a mold process or an uneven cooling applied after the removal from the mold tool. In the optical scanning system, many optical devices typically have extended lengths in the main scanning direction. These optical devices are apt to bend in the sub-scanning direction. This can cause a deviation of dot positional in the sub-scanning direction if an optical device is not held in a preferable manner.
FIG. 4 shows a displaced scanning line due to the deviation of dot position formed on the photosensitive member 7. In FIG. 4, L1 represents a preferable scanning line and L2 represents a displaced scanning line.
When the optical devices are mounted to the optical housing, cumulative errors in the mounting positions can often occur and they become a non-negligible deviation in dot position in the sub-scanning direction on the photosensitive surface. This phenomenon is a crucial drawback, particularly, for a tandem-type full color copying apparatus. Generally, a tandem-type full color copying apparatus is provided with an optical scanning system and four photosensitive drums serving as image carrying members for carrying images of cyan (C), magenta (M), yellow (Y), and black (K) colors, arranged along with a sheet transfer surface of a transfer belt. The multi-beam optical scanning system forms latent images of cyan, magenta, yellow, and black colors with a plurality of laser light beams on the respective surfaces of the photosensitive drums. Then, the latent images are individually developed into visual images with respective color toners and are sequentially transferred onto a surface of a recording sheet in an overlaying fashion. Thus, if dot positions of the latent images for the cyan, magenta, yellow, and black colors are deviated in the sub-scanning direction, a phenomenon of color deviation occurs and accordingly image quality is degraded.
An important point to improve the quality of a color image created by such a full color copying machine handling multiple colors as described above is precise adjustment of the scanning lines of the multiple laser light beams for the respective color images to the same position to perfectly overlay each other. Errors in scanning line positioning occur mainly because of deviations of the multiple laser light beams in the sub-scanning direction. Such deviations are apt to be caused by deformed molded plastic optical devices since, as noted above, many optical devices are provided as molded plastic materials to reduce the cost of manufacturing.
Japanese Laid-Open Patent Publication No. 10-268217 describes an optical scanning system which provides a slit-shaped plastic lens with each set of lenses for a light beam. In this system, the slit-shaped plastic lens is previously bent in the sub-scanning direction to attempt to correct for a bend of the scanning line. However, in this attempt, relative positions between the scanning lines in the sub-scanning direction, that is, the scanning line pitch, are not considered.